The ability of muscle to redistribute the energy of mechanical shock in time and space and unload skeletal joints is of key importance in physical activities, such as running and jumping, playing soccer, wrestling, etc. The impacts that occur during such activities may exceed the level that can be tolerated by cartilage pads between joints which cushion the moderate impacts accompanying mundane activities such as walking. We hypothesize that dissipation of mechanical energy of external impacts to absorb mechanical shock is a fundamental function of skeletal muscle and the viscosity of the skeletal muscle is a variable parameter which can be voluntarily controlled by changing the tension of the contracting muscle. We further hypothesize that an ability of muscle to absorb shock had been developed in the course of biological evolution, allowing the life to move from the ocean to land, from hydrodynamic to aerodynamic environment with dramatically different loading conditions for musculoskeletal system. To date there are no available practical tools for assessment of muscle dynamic viscosity. We propose to develop a compact and easy-to-use instrument for assessment of viscoelastic properties of skeletal muscle in vivo that can be used in exploring the mechanism and the ability of muscle in reducing the harmful effect of external impacts on bones and joints. The device will be based on the use of an acoustic radiation force impulse to generate localized displacements in tissue and the resulting displacements will be tracked during relaxation using cross-correlation based methods. The goals of this project include bench-testing of the developed device on muscle phantoms and conducting preliminary pilot study on a limited group of athletes demonstrating feasibility of the proposed technology. Experimental data obtained in vivo will be compared with theoretical predictions of the mathematical model for the molecular mechanism of the muscle viscosity. Quantitative assessment of muscle viscosity may shed light to many problems of medical biomechanics and sports medicine. The list of muscle disorders that can be assessed by a device for quantitative assessment of muscle viscoelasticity includes muscle dystrophy, motor neuron diseases, inflammatory and metabolic myopathies and many more. Monitoring muscle viscosity could help in anticipating joint diseases, such as tendonitis, bursitis and osteoarthritis and decreasing the probability of osteoporotic bone fracture. The fields of potential applications of such device include general clinical applications; rehabilitation and occupational medicine; sports medicine: assessment of muscle condition and performance in athletes; gerontology: assessment of muscle changes in aging, particularly following total weakening of musculoskeletal system during osteoporosis and osteoarthritis, and estimation of the prophylactics efficacy.